The Truth About Toilets

Ever wondered about that implausible-sounding Coriolis Effect business? Well, I have. And thanks to the magic of Adium and Brian Beutler's trip to Argentina we were able to conduct a little experiment:

So, having been disabused of the Coriolis disinformation for a few years now, Matt and I conducted a (wasteful) experiment. It turns out that when you fill up and drain my kitchen sink here in Buenos Aires it drains clockwise. AND, perhaps evidince of swirl-rumor confirmation, Matt's toilet flushes counter clockwise. But nothing's ever that simple. Because my toilet down here flushes counter clockwise as well.

I have no idea how regular hemisphere-wide toilet-flush trends are, and if there is a physical explanation, I have no idea what it is. My guess would be that swirl direction is purely a function of how the relevant pipes are structured. But I'll leave that to the blogosphere (which I know to be a trustworthy and mature resource) to decide.

Take that, science!

Comments

My understanding is that the water swirls in an essentially random direction, based on whatever turbulence/Brownian motion happened to be present in the water when it began draining. Plus I think toilets are manufactured with the jets pointing in a given direction, forcing the water to swirl that way.

Posted by OhioBoy | April 2, 2007 5:59 PM

The Coriolis effect is too weak over such short distances as a toilet bowl diameter.

But anyway, didn't the Simpsons already deal with this? I thought the American embassy in Australia installed toilets to make them swirl the correct American way.

Posted by matt | April 2, 2007 6:02 PM

The direction the water swirls is based on the geometry of the bowl or sink and its orientation to gravity. There velocity of the water is too slow for the coriolis effect to have an effect on the swirl. I learned this in my "continuum physics" couse at Cornell and have verified it in New Zealand and Australia. Toilets are not affected by a change in hemisphere, but artillery trajectories are, as the British learned in the Falklands War.

Posted by fostert | April 2, 2007 6:04 PM

thinking the coriolis effect is going to have an impact on your household drains is like thinking the expanding universe is making the walk to work just that much longer each day... or getting dizzy from orbiting the sun.

Posted by foo | April 2, 2007 6:05 PM

http://www.snopes.com/science/coriolis.asp

Posted by Freddie | April 2, 2007 6:06 PM

However, in the Southern Hemisphere, the moon is upside down. Now that's weird.

Posted by neil | April 2, 2007 6:19 PM

What I don't understand is all the talk of the storm on Saturn's north pole. I mean how do they know its the north and not the south one? Blatant northernerism if you ask me.

Posted by Rob | April 2, 2007 6:23 PM

Well this experiment proves little, because we don't know if your sense of clockwise flips as well when crossing the equator.

Posted by jerry | April 2, 2007 6:23 PM

And in Rand McNally people where shoes on their heads and hamburgers eat people.

Posted by UberMitch | April 2, 2007 6:23 PM

http://www.newton.dep.anl.gov/newton/askasci/1993/general/GEN016.HTM

Tornado Spinning

Question: Why do tornadoes turn in a counter-clockwise direction in the
northern hemisphere and clockwise in the southern hemisphere? Is it magnetic?
---------------------------------------
Not to complicate things, but I am reasonably sure that tornadoes
in the Northern Hemisphere have been observed to spin in both directions,
although one is more prevalent...

Mark Fernau
====================================================================
At least the great majority of tornadoes rotate counterclockwise
(as do all low-pressure systems) in the northern hemisphere, and clockwise in
the southern hemisphere, for the reason (coriolis force) given by Eric
Peterson in response #1. But occasionally, it would seem, northern hemisphere
tornadoes do rotate clockwise: S. Flora's book "Tornadoes of the United
States" cites an 1890 article in the American Meteorological Journal. Its
author, a J.P. Finley, states that, of 550 American tornadoes he studied, 29
were deemed to have rotated clockwise. I have not been able to find any
"modern" study of this question. But I believe it could be true. The region
of swirling air that contracts to become the tornado is not itself large
enough in extent to have its rotation dictated by the coriolis force; rather,
it "inherits" this tendency from the great masses of air whose movement sets
the stage for the storms and any associated tornadoes. If the study cited is
correct and representative, on occasion the direction of rotation is set by
some other factor, perhaps the topography in the area where the tornado forms,
for example.

Ronald Winther
====================================================================
The previous responses were correct in that the coriolis force is
the reason air circulates in a clockwise direction around high pressure and
counterclockwise around low pressure in the northern hemisphere. The coriolis
force can explain the rotation in large scale high and low pressure areas
including hurricanes. However, the rotation of a tornado is much more
complicated. Tornadoes in the northern hemisphere can rotate in either
direction but counterclockwise rotation is much more common. The rotation is
produced by wind shears and pressure forces in and near the parent
thunderstorm. Thunderstorms form when warm moist air rises rapidly upward.
This upward current of air within a thunderstorm is referred to as an updraft.
If sufficient vertical wind shear exists, this updraft will rotate. Vertical
wind shear is a change in wind speed and/or direction from the ground up
through the atmosphere. If the wind speed increases rapidly with height
and/or if the wind direction turns clockwise with height, air being drawn in
toward the thunderstorm updraft will develop a spin about the horizontal axis.
Think of an imaginary paddlewheel floating in the air. If winds blowing
across the top of the paddlewheel are stronger than the winds at the bottom of
the paddlewheel, it will spin. As the air rises into the updraft, the spin
about the horizontal axis becomes a spin about the vertical axis. Imagine if
you take a rope and roll it along the ground (it is spinning about the
horizontal). Now pick up the middle of the rope but keep rolling it. Now the
two dangling ends are spinning about the vertical. But one end is turning
clockwise and the other counterclockwise. Strong winds blowing through the
storm produce pressure forces within the storm that enhance or suppress the
updraft. Most tornadoes in the United States occur in the warm humid air mass
ahead of an approaching low pressure area. Because of the coriolis force,
winds usually turn clockwise with height. This wind profile enhances the
counterclockwise rotating updraft and suppresses the clockwise portion of the
updraft. That is why most tornadoes turn counterclockwise. However if winds
are from a nearly uniform direction throughout the depth of the storm, both
circulations can be maintained. In this case the storm can split producing
both a clockwise and anti-clockwise rotating tornadoes. This has been
documented with radar.

Jim Allsopp

Posted by jerry | April 2, 2007 6:28 PM

Although I have a fondness for any myth cited in the song "Ana Ng," I have known for some time that this is false. Despite your "Take that, science!" scientists would actually be the ones most pleased to see the myth refuted once again.

Posted by James Kabala | April 2, 2007 7:05 PM

These bloody things are everywhere! They're in the lift, in the lorry, in the bond wizard, and all over the malonga gilderchuck!

Posted by norbizness | April 2, 2007 7:31 PM

With all our 21st century technology (based, in its current form at least, on Thomas Crapper's contributions in 19th century England - including a colloquial word for the device and a shortened form usually applied to anything written by Jonah Goldberg), it seems as if we could have a toilet of choice. Do you want a flush left or flush right?

How about a solution that gives the user's desired flush direction (two handles or a button to set the choice), so conservatives could flush to the right, and liberals could flush to the left?

However, this choice possibility does raise some issues for genuine independents and libertarians which surely would disavow either a right-flush or left-flush. Maybe a 'let it mellow' option is all they require.

Posted by JimPortlandOR | April 2, 2007 7:41 PM

Ah, I see you've played Knifey-Spoony before!

Posted by Bruce | April 2, 2007 7:43 PM

I have a theory of Cioriolis forces, which is mine.

Posted by A. Elk | April 2, 2007 7:55 PM

It just so happens that I'm the author of the Physics FAQ article on this very topic,.. yes, the world's expert on the effect of geophysical forces on bathroom fixtures...

Posted by MattF | April 2, 2007 8:21 PM

Under the rim there are several holes, all bored so as to give a direction to the flow. It is not unknown for these holes to lime up, and the swirl is lessened as a result. Keep your swirl holes clean! This has been a public service announcement.

Posted by jeffreyw | April 2, 2007 8:26 PM

As long as I'm revisiting the distant past, here's the original version of the FAQ article, before it was mushed up by people with a weak grasp of English grammar:

Question: Does my bathtub drain differently depending on whether I
live in the northern or southern hemisphere?

Answer: No.

Because the earth rotates, a fluid that flows along
the earth's surface feels a "Coriolis" acceleration perpendicular to its
velocity, but this acceleration is --very-- weak for bathtub-scale
fluid motions. The order of magnitude of the Coriolis acceleration
can be estimated from size of the "Rossby number".

If
Omega = angular velocity
L = length scale
U = velocity scale
then the Rossby number is
= U/(2*L*Omega)
Coriolis accelerations are significant when the Rossby number is
--small--.

So, suppose we want a Rossby number of 0.1 and a bathtub-vortex
length scale of 0.1 meter. Since the earth's rotation rate is about
10^(-4)/second, the fluid velocity should be less than or equal to
2*10^(-6) meters/second.

This is a very small velocity. How small is it? Well, we can
take the analysis a step further and calculate another, more famous
dimensionless parameter, the Reynolds number.

The Reynolds number is = L*U*density/viscosity

Assuming that physicists bathe in hot water the viscosity will be
about 0.005 poise and the density will be about 1.0, so the Reynolds
Number is about 4*10^(-2).

Now, life at low Reynolds numbers is different from life at high
Reynolds numbers. In particular, at low Reynolds numbers, fluid
physics is dominated by friction and diffusion, rather than by
inertia: the time it would take for a particle of fluid to move a
significant distance due to an acceleration is greater than the time
it takes for the particle to break up due to diffusion.

Therefore the effect of the Coriolis acceleration on your bathtub
vortex is --ZERO--.

Posted by MattF | April 2, 2007 8:35 PM

I tested this out when I was in New Zealand. I filled up the sink, let the water settle and opened the drain. With perfect consistency the water spiraled one way (I don't remember which) when I filled it from the hot faucet and the other when I filled it from the cold faucet. Of course, the choice of faucet must have caused an imperceptible flow that influenced the direction it spiraled, but I was amused.

Posted by interrobang | April 2, 2007 9:19 PM

When I was in college (beer may have been involved, but it's also quite possible we did this sober) we tested this in the big laundry room sinks. It turned out we could easily make it go one way or the other by helping it along with hand swirling at the beginning of the process. Left to itself, it would go either way but we didn't do it enough for a decent estimate of the probabilities.

Posted by fizz | April 2, 2007 9:28 PM

Forget the Coriolis effect. What is stunning in the Southern Hemisphere are the constellations you cannot see in the Northern hemisphere.

Posted by Randy Paul | April 2, 2007 10:27 PM

Sinks and toilets really DO drain in the opposite direction down there in the southern hemisphere. Observers who claim to the contrary are failing to allow for the fact that they are standing upside down . . .

Posted by rea | April 2, 2007 10:46 PM

For Christ's sake, man, you act as if you lived in an age before the internet. Cecil addressed this in 1983.

http://www.straightdope.com/classics/a1_161.html

Posted by C. | April 3, 2007 12:05 AM

Having a case of whimsy ? Sounds good.

Posted by opit | April 3, 2007 1:39 AM

C. - Always good to see someone citing the Perfect Master.

Posted by RT | April 3, 2007 5:10 AM

As an MIT graduate, I can only say: this is the perfect post for a Harvard graduate.

Posted by mpowell | April 3, 2007 2:36 PM

Learned in grad school how the Coriolis Force should be imperceptible in something as small as a toilet or sink. But I couldn't resist an experiment. I drained a sink in Australia. The drain was so large the water went straight down: no swirl either way....

Posted by Marc Valdez | April 4, 2007 3:53 AM